The invention relates to synchronization of a receiver with a signal to be received.
In CDMA (Code Division Multiple Access), a narrowband data signal of a user is modulated to a relatively broad band by a spreading code which is more broadband than the data signal. The known CDMA systems use bandwidths over 1 MHz. In a WCDMA (Wide-band CDMA) radio system, the bandwidth is even much more greater since the current mobile telephone networks aim to provide the user with increasingly versatile services.
The spreading code of the CDMA radio system usually comprises a long pseudo-random bit sequence. The bit rate of the spreading code is much higher than that of the data signal and, as distinct from data bits and data symbols, the bits of a spreading code are called chips. Each data symbol of the user is multiplied by the chips of the spreading code. The narrowband data signal thus spreads to the frequency band used by the spreading code. Each user has a unique spreading code. A spreading code may be one or more data bits long. A plurality of users transmit simultaneously in the same frequency band, and the data signals are separated in the receivers on the basis of the pseudo-random spreading code.
In spread-spectrum systems, such as the CDMA radio system, a receiver should synchronize with the signal to be received in order for enabling signal modulation and detection. Code synchronization is usually divided in two parts: code acquisition and code tracking. In code acquisition, the timing difference between the spreading code of a code generator of the receiver and the spreading code of the received signal is to be no more than one chip of the spreading code. To achieve this, the spreading code of the code generator of the receiver is delayed chip by chip, searching for the delay between the signal spreading code and the code generator spreading code. Code acquisition is the most challenging task in spread-spectrum receivers. In code tracking, in turn, the aim is to keep the timing inaccuracy as low as possible, i.e. fractions of a chip.
Code acquisition is carried out e.g. using a simple correlator. When the spreading code of the signal to be received and the spreading code of the code generator of the receiver are not synchronized, the output of the correlator is provided with a low value. But when the spreading code of the signal to be received and the spreading code of the code generator of the receiver are synchronous, i.e. the delay of the spreading code of the code generator of the receiver is a correct one, the output value of the correlator is high. Code acquisition can also be carried out using a matched filter. A threshold detector is typically connected to an output of the matched filter to indicate that synchronization has been found.
Radio telecommunication connections are susceptible to interference. For instance, multiple access interference MAI, noise and multipath propagation cause great temporary values at the output of the correlator even if the spreading code of the signal to be received and the spreading code of the code generator of the receiver were asynchronous. This results in a false alarm and prolonged code acquisition. In order to reduce the effect of false alarms, the delay obtained in code acquisition should be verified by a second correlation measurement, which results in a longer synchronization time.
An object of the invention is to provide an improved method for carrying out synchronization, and a receiver whereto the method can be applied. This is achieved by a method for carrying out code synchronization when a signal has been spread-coded by a pseudo-random spreading code. The method comprises forming angular cells, determining interference power in each angular cell, arranging the angular cells according to the interference power, carrying out code acquisition by examining the arranged angular cells by using different delays of the spreading code, starting from the angular cell with the lowest interference power.
The invention further relates to a method for carrying out code synchronization when a signal has been spread-coded by a pseudo-random spreading code. The method comprises forming angular cells, determining interference power in each angular cell, determining user equipment location probabilities in each angular cell, determining a code acquisition order for the angular cells by utilizing an interference power estimate and location probability information on the user equipment, carrying out code acquisition by examining the angular cells in chosen order.
The invention further relates to a method for carrying out code synchronization when a signal has been spread-coded by a pseudo-random spreading code. The method comprises forming angular cells, determining user equipment location probabilities in each angular cell, determining a code acquisition order for the angular cells by utilizing location probability information on the user equipment, carrying out code acquisition by examining the angular cells in chosen order.
The invention further relates to a receiver implementing the method, the receiver being arranged to carry out code acquisition from a signal to be received, the signal having been spread-coded by a pseudo-random spreading code. The receiver comprises means for forming angular cells, the receiver comprises means for determining interference power in each angular cell, the receiver comprises means for arranging the angular cells according to the interference power, the receiver comprises means for carrying out code acquisition by examining the arranged angular cells by using different delays of the spreading code, starting from the angular cell with the lowest interference power.
The invention further relates to a receiver arranged to carry out code acquisition from a signal to be received, the signal having been spread-coded by a pseudo-random spreading code. The receiver comprises means for forming angular cells, the receiver comprises means for determining interference power in each angular cell, the receiver is arranged to determine user equipment location probabilities in each angular cell or the receiver is arranged to utilize location probability information on the user equipment, the receiver comprises means for determining a code acquisition order for the angular cells by utilizing an interference power estimate and the location probability information on the user equipment, the receiver comprises means for carrying out code acquisition by examining the arranged angular cells in chosen order.
The invention further relates to a receiver arranged to carry out code acquisition from a signal to be received, the signal having been spread-coded by a pseudo-random spreading code. The receiver comprises means for forming angular cells, the receiver is arranged to determine user equipment location probabilities in each angular cell or the receiver is arranged to utilize location probability information on the user equipment, the receiver comprises means for determining a code acquisition order for the angular cells by utilizing the location probability information on the user equipment, the receiver comprises means for carrying out code acquisition by examining the arranged angular cells in chosen order.
Preferred embodiments of the invention are disclosed in the dependent claims.
The idea underlying the invention is that code acquisition is carried out two-dimensionally, the dimensions being time dimension and angular dimension. Code acquisition utilizing the angular dimension is carried out by using several parallel antenna beams, i.e. angular cells. The interference level of the angular cells, i.e. interference power (also called interference level), is determined and the angular cells are arranged preferably as a vector wherein the order of elements is determined according to the interference power. Interference power also comprises noise. Code acquisition starts from the angular cell with the lowest interference power. An alternative is to determine user equipment location probabilities and to arrange the angular cells such that code acquisition is first carried out in the angular cell in which the user equipment is located most likely, and last in the angular cell in which the user equipment is located least likely. An alternative is also to combine these angular cell arrangement methods. Code acquisition then starts from the cell in which the interference level and the location probability are optimized, e.g. code acquisition starts from an angular cell with a low, but not necessarily the lowest, interference level if it is highly probable that the user equipment is located in this angular cell.
The method and system of the invention provide several advantages. Code acquisition can be made more efficient in an environment containing interference particularly when the interference levels vary in different angular cells. The method and system of the invention enable the time spent on code acquisition to be shortened.